1. Field of the Invention
The present invention relates generally to implantable cardiac pacemakers, and more specifically to rate-responsive pacemakers wherein the upper rate is limited as the battery approaches its end-of-life (EOL). In alternative embodiments, the invention can be used with any high power consumption features within an implantable device to extend the longevity of the battery by limiting the extent to which these features may be utilized.
Implantable cardiac pacemakers are powered by a battery within the pacemaker housing. Once implanted, it is difficult to determine the battery's state of depletion and, thus, the need for replacement. Although the surgery required for replacement is relatively minor, the associated risks of complications to the patient are ever present. In general, it is considered better to avoid replacement of a properly functioning pacemaker until absolutely necessary.
To determine when to explant a pacemaker prior to its EOL, physicians plan their follow-up schedules less frequently during the battery's "beginning-of-life" (BOL) and more frequently towards the battery's recommended replacement time (RRT) and the battery's "end-of-life" (EOL). (EOL is defined as the point in time in which the pacemaker pulse amplitude is reduced to approximately 50 percent of the programmed value.) As the basis, physicians estimate the remaining battery capacity by subtracting the "nominal" current drain of the pacemaker, usually specified at 5 volts with 100% pacing at a rate of 70 pulses-per-minute (ppm), from the theoretical available amp-hour capacity of the battery. Even though accurate battery capacity sensors have been developed (see, for example, U.S. Pat. No. 4,556,061 to Barreras et al.), the physician must still accurately predict the power consumption for the remaining period. With sophisticated pacemakers and unpredictable current drain modes of operation, physicians have to schedule more frequent follow-up visits to accurately monitor the replacement time and still avoid premature surgical replacement.
Current drain on a battery is largely dictated by the pacer output amplitude, pulse width, and rate. Programmability of these pacemaker parameters offers some flexibility to safely prolong the longevity of the battery. For example, it is well known that the battery life can be increased anywhere from 3 to 9 months by programming the rate to 70 instead of 90 beats-per-minute (bpm). However, not all patients can tolerate being paced at 70 bpm. Active patients need a higher rate during exercise. In patients with a normal sinus node, higher rates may be achieved with a dual chamber pacemaker, wherein the atrial rate is sensed and the ventricles are stimulated a short delay later (mimicking a normal heart). During exercise, the atrial rate may vary between 70 and 120 bpm or more. It is also known that rate-responsive pacemakers can increase the pacing rate according to an additional sensor (accelerometer or "activity" sensor, oxygen saturation, QT measurements, respiration rate, temperature, etc.). The purpose of such pacemakers is to accelerate the rate when the atrium is incompetent, that is, non-responsive to exercise stress or prone to atrial flutter or fibrillation.
In both of these pacemakers, the amount of current drain on the battery can change quite rapidly as the pacing rate of the pacer may change from a low rate to a high rate. This is especially true where the patient's own intrinsic rhythm is able to sustain the patient's needs at low activity levels (a low current drain condition), but where stimulated pacing is required in one or both chambers of the heart at a high activity level (a high current drain condition). Unfortunately, such large variations in current drain can cause a sudden battery voltage drop below the EOL voltage level such that the possibility exists that the battery voltage could drop low enough to cause loss of capture. Furthermore, if pacing occurs at fast rates, such as occurs during exercise, the increase in current drain could dramatically reduce or even eliminate the safety margin associated with the last reported recommended replacement time (RRT) of the pacer, particularly when the last reported RRT is based on the current drain while the patient was at the rest rate.
It is also known in the art (see for example, U.S. Pat. No. 4,686,988 to Sholder) that battery current drain due to the delivered pacing pulse can be reduced by automatically adjusting the output amplitude and/or pulse width of the pacing pulse such that the lowest possible output is delivered which can still stimulate or "capture" the heart. This feature does ensure that the patient will not lose capture throughout the life of the pacemaker, however, this increase in processing time of the microprocessor and the constant changing of the output amplitude and/or pulse width introduces still more variables to consider when determining the replacement time of the pacemaker.
Furthermore, with the advent of microprocessor-based pacemakers, functionality has been extended to automatic adjustment of pacemaker parameters, storing and telemetering of intracardiac electrograms (EGMs), processing multiple sensors, detecting and breaking arrhythmias and recognizing waveform patterns. The current drain of the pacemaker may also be significantly influenced by the duty cycle of the microprocessor in performing these functions. Without careful monitoring of the battery voltage, these high current drain situations may cause a temporary drop in available battery voltage, increase the risk of loss of capture, and dramatically use up the remaining battery capacity.
What is needed is a pacemaker which can regulate its own current drain usage, conserve the limited battery energy towards EOL, prevent loss of capture by limiting high current drain modes, and ultimately eliminate premature replacement of the pacemaker by eliminating the unpredictable nature of the RRT to EOL interval. Furthermore, this pacemaker should not burden the physician by increasing the number of follow-up visits near EOL.